Esa Räikkönen

Supercontinuum generation by nanosecond dual-wavelength pumping in microstructured optical fibers

We study experimentally the spectral evolution of supercontinua in two different microstructured fibers that are pumped with nanosecond pulses from dual-wavelength sources of either 1064/532 nm or 946/473 nm output. The experimental findings are compared with simulations based on numerically solving the nonlinear Schrödinger equation. The role of cascaded cross-phase modulation processes and the group-delay properties of the fiber are emphasized and demonstrated to determine the extent of the broadening of the continua to the visible wavelengths.

Modeling the Time-Dynamics of Miniature Passively

The pulse-buildup in miniature passively Q-switched lasers occurs in a similar time-scale as the phonon-assisted thermalization of the laser manifolds. We study how the time-dynamics of the laser is affected by the thermalization and relaxation processes using a dimensionless geometrical rate equation model for a homogeneously broadened four-level gain medium. The pulse length is found to depend strongly on the thermalization rate when the ratio of the ground-state absorption cross section of the saturable absorber to the spectroscopic emission cross section of the laser transition is small (lsim 10). Numerically calculated design curves are shown for the 1064 nm transition in Nd:YAG, and guidelines are given for applying the model to other transitions. Experimental results are used for estimating the thermalization time constant of Nd:YAG for the 1 mum transition group and the ground-state absorption cross section of the Cr4+:YAG saturable absorber crystal.